1. Field of the Invention
The present invention relates to a semiconductive roller and a method of manufacturing the semiconductive roller.
2. Description of the Related Art
With a conventional electrophotographic printer, a charging roller charges the surface of the photoconductive drum and an exposing unit such as an LED head writes an electrostatic latent image on the charged surface of the photoconductive drum. The electrostatic latent image is then developed with toner into a visible image, i.e., toner image. The toner image is subsequently transferred to a print medium when the print medium passes in a sandwiched relation between the photoconductive drum and a transfer roller. The transfer roller receives a high voltage of about several hundred to several thousand volts and of an opposite polarity to the toner. An electric field developed between the photoconductive drum and the transfer roller causes the toner image to be attracted to the print medium. Thus, the toner image is transferred to the print medium.
Some of the toner fails to be transferred from the photoconductive drum to the print medium. Such residual toner is distributed evenly on the surface of the photoconductive drum by a cleaning unit. A developing unit subsequently collects the evenly distributed residual toner.
A toner cartridge supplies fresh, unused toner into a toner container, which in turn supplies the toner to the developing roller via an agitating bar and a semiconductive sponge roller. When the sponge roller transports the toner, the toner is charged. The sponge roller is in the form of a conductive silicone rubber that contains silicone polymer and carbon as a conductive agent. The conductive silicone rubber is foamed to have a large number of holes referred to as cells having diameters in the range from 0.3 to 0.5 mm. The sponge roller serves to ensure required print density and prevent variations in print density over time. The sponge roller is incorporated in a print process unit, which is a mechanical section of an electrophotographic printer.
The problem with the aforementioned conventional sponge roller is that as the cumulative number of printed pages increases, the carbon contained in the material tends to clump, increasing conductivity. An increased conductivity decreases the electrical resistance of the sponge roller. As a result, a larger current flows through the sponge roller. This results in a steep increase in toner potential so that more toner than necessary is supplied to the developing roller, especially to longitudinal ends of the developing roller which have relatively lower electrical resistance. Thus, excessive toner falls in the electrophotographic printer so that the print medium opposing the longitudinal ends of the developing roller becomes black. The toner deposited on the developing roller forms a "ring" that surrounds the developing roller, and is referred to as "toner ring" in this specification.